Surface Dilatational Modulus Research Articles

Liquid films with high surface modulus moving in tubes: dynamic wetting film and jumpy motion

We investigate the motion of transverse soap films, or lamellae, of high surface dilatational modulus through a wet tube. Combining local thickness and velocity measurements in the wetting film, we reveal a zone several centimeters in length, the dynamic wetting film, which is significantly influenced by a moving lamella. The dependence of this influence length on lamella velocity and wetting film thickness provides discrimination among several possible surfactant minimal models. A spectacular jumpy mode of the unsteady motion of a lamella is also evidenced.

Effect of Cationic Polymers on Foam Rheological Properties

We study the effect of two cationic polymers, with trade names Jaguar C13s and Merquat 100, on the rheological properties of foams stabilized with a mixture of anionic and zwitterionic surfactants (sodium lauryloxyethylene sulfate and cocoamidopropyl betaine). A series of five cosurfactants are used to compare the effect of these polymers on foaming systems with high and low surface dilatational moduli. The experiments revealed that the addition of Jaguar to the foaming solutions leads to (1) a significant increase of the foam yield stress for all systems studied, (2) the presence of consecutive maximum and minimum in the stress vs shear rate rheological curve for foams stabilized by cosurfactants with a high surface modulus (these systems cannot be described by the Herschel-Bulkley model anymore), and (3) the presence of significant foam-wall yield stress for all foaming solutions. These effects are explained with the formation of polymer bridges between the neighboring bubbles in slowly sheared foams (for inside foam friction) and between the bubbles and the confining solid wall (for foam-wall friction). Upon addition of 150 mM NaCl, the effect of Jaguar disappears. The addition of Merquat does not noticeably affect any of the foam rheological properties studied. Optical observations of foam films, formed from all these systems, show a very good correlation between the polymer bridging of the foam film surfaces and the strong polymer effect on the foam rheological properties. The obtained results demonstrate that the bubble-bubble attraction can be used for efficient control of the foam yield stress and foam-wall yield stress, without significantly affecting the viscous friction in sheared foams.

Surface dilational moduli of poly(vinyl acetate), poly(methyl methacrylate), and their blend films spread at the air–water interface

Surface dilational moduli of poly(vinyl acetate) (PVAc), poly(methyl methacrylate) (PMMA), and PVAc/PMMA blend films spread at the air–water interface were investigated as a function of surface concentration. Moreover, the surface dilational moduli of the PVAc/PMMA blend films were measured as a function of the molar fraction of PVAc for the first time. The surface dilational modulus of an expanded PVAc film increased as the surface concentration increased up to 0.75 mg/m 2, which corresponded to the limiting surface area of PVAc. After peaking at this value, the surface dilational modulus decreased with increasing PVAc surface concentration. The surface dilational modulus of a condensed PMMA film increased steeply as the surface concentration increased from 1.00 to 1.45 mg/m 2. The magnitude of the surface dilational modulus of PMMA was larger than that of PVAc at a fixed surface concentration. This difference was primarily attributed to the fact that PMMA is a condensed polymer whereas PVAc is an expanded polymer. The surface dilational moduli of the incompatible PVAc/PMMA blend films at fixed surface pressures decreased and approached those of PVAc as the molar fraction of PVAc increased, irrespective of the surface pressure. These findings suggest that PVAc components in the blend films are preferentially adsorbed at the air–water interface.

Interfacial and foaming properties of bovine β-lactoglobulin: Galactose Maillard conjugates

In this paper, the effect of the initial and advanced steps of glycosylation by Maillard reaction (MR) (glycation) of β-lactoglobulin (β-Lg) with galactose on the interfacial and foaming (foamability and foam stability) properties of this protein has been studied at both pH 7 and pH 5. Hardly any effect of glycation was observed at pH 7. However, a pH 5, due to its increased solubility, β-Lg glycated at 50 °C during 48 h (advanced steps of MR) presented the best dynamic of adsorption which lead to an increase of the surface dilatational modulus of adsorbed film. This resulted in a better foaming capacity, as well as higher stability of foams of β-Lg glycoconjugates with respect to native and control heated protein. These results could extend the applicability of β-Lg as a foaming agent, particularly in acid foods.

Pendant drop tensiometry for the evaluation of the foaming properties of milk-derived proteins

Aim of the present study was to investigate the relationship between parameters derived from pendant drop tensiometry and foaming properties of milk-derived proteins. Foaming time and foam stability of solutions of whey protein isolate and two different whey protein hydrolysates with varying protein content were analysed. Pendant drop tensiometry was used to determine the surface elasticity and different parameters from a modified dynamic surface tension measurement to characterise protein adsorption to the air-water interface. A modified pendant drop technique allowed the characterisation of the surface occupation of proteins for very fast processes like air bubble stabilisation during foaming. Principal component analysis revealed a close relation between foaming time and parameters from dynamic surface tension measurement (lag time for protein adsorption to the interface, slope of the regression line for the change in surface tension) as well as foam stability with parameters derived from interfacial rheology (surface dilatational modulus as well as elastic and viscous components, phase angle). Therefore pendant drop tensiometry proved to be a valuable tool for the characterisation and prediction of the foaming properties and foam stability of protein solutions.

Surface Dilatational Moduli of Poly(vinyl acetate) (PVAc) and PVAc-Poly(n-hexyl isocyanate) (PHIC) Blend Films at the Air–Water Interface

Surface dilatational moduli of poly(vinyl acetate) (PVAc) film and blend films of PVAc and poly(n-hexyl isocyanate) (PHIC) were measured at the air-water interface. PVAc formed a film that was looser and also more stable against strain than the PHIC film. The apparent surface dilatational modulus and surface pressure of the blend films were superimposed on the lower concentration of PVAc, irrespective of the composition of PVAc. However, the additivity rule was not applicable to the apparent surface dilatational modulus and surface pressure. The scaling exponents of the apparent surface dilatational modulus against the added surface concentration decreased with an increase in the proportion of PVAc, suggesting that blend films gradually change from glass material to expanded films.

Dilational Properties of Sodium 2,5-Dialkyl Benzene Sulfonates at Air−Water and Decane−Water Interfaces

The dilational properties of adsorbed film of sodium 2,5-dialkyl benzene sulfonates at the air−water and decane−water interfaces have been investigated by a Langmuir trough method. The influences of oscillating frequency and bulk concentration on dilational modulus and phase angle were expounded. The experimental results showed the hydrophobic chains located at different positions have different effects on dilational properties: for the air−water interface, the short alkyl chain ortho to sulfonate group produces a small increase in dilational modulus, while an increase in long para-alkyl plays an important role in the surface relaxation process, which leads to a higher surface dilational modulus. In the case of the decane−water interface, the interfacial dilational moduli are obviously lower than those of the air−water interface due to the insertion of decane molecules from oil phase. Moreover, the strong interactions between para-alkyl chains were weakened by decane molecules; therefore, the interfacial ...

A Surface Rheological Study of Polyoxyethylene Alkyl Ether Carboxylic Salts and the Stability of Corresponding Foam

The dynamic dilational viscoelastic properties of polyoxyethylene alkyl ether carboxylic salts at the air-water interface were investigated by drop shape analysis method and their foam stability were measured by Bikerman Method. The influences of time, dilational frequency, and bulk concentration on surface dilational modulus and phase angle were expounded. The results show that the surfactant with the longest straight-chain shows the highest dilational modulus, which in agreement with the best foam stability. However, the foam stability of branched-alkyl chain surfactant cannot be explained in terms of film elasticity alone.

Effects of electrolyte and nonelectrolyte additives on adsorption and surface rheological characteristics of humic acid salt solutions

The du Nouy and oscillating droplet shape methods are employed to study the effects of the ionic strength and pH of a medium, as well as the addition of nonelectrolytes (lower alcohols and acetone), on the adsorption and surface rheological characteristics of aqueous solutions of humic acid salts (sodium humates) at the liquid-air interface. When added in concentrations at which the aggregation of humic substances is not yet observed, strong electrolytes (NaCl and HCl) decrease the equilibrium surface tension and increase the dilatational viscoelastic modulus of aqueous sodium humate solutions. The aggregation of humic substances enhances the surface tension and reduces the viscoelastic modulus of surface layers. Nonelectrolyte additives decrease the surface tension and dilatational modulus of aqueous humic acid salt solutions. The equilibrium surface tension of sodium humate-nonelectrolyte mixed solutions is described in terms of two different models, namely, a relatively exact model of polyelectrolyte-nonionic surfactant adsorption and a simple additive model. It is shown that the additive model may be used to predict the equilibrium surface tension for the mixtures of high- and low-molecular-mass surfactants.

Study on Foaming Properties of Polyoxyethylene Alkyl Ether Carboxylic Salts with Different Structures

In this article, foaming properties, dynamic surface tension, and surface dilational properties were carried out with aqueous solutions of polyoxyethylene alkyl ether carboxylic salts to elucidate the relationship between foaming properties and surfactant structures. It was found that the dynamic surface tension of the solution is the key factor in determining the foamability: with increasing in the length of the hydrophobic group, dynamic surface activity becomes low, which leads to the low foam volume; the surfactant with branched-alkyl chain has faster diffusion rate, which lead to higher dynamic surface activity and higher foam volume. However, for straight chain surfactant, the foam stability is correlated with the surface dilational properties: the change rule of surface dilational modulus is consistent with that of the foam stability.

Surface Dilational Properties and Foam Properties of Novel Benzene Sulfonate Surfactants

The dilational viscoelaticity properties of octyl-[ω-alkyloxy-tetraoxyethylene]yl-benzene sulfonates at the air/water interface were investigated by means of oscillating barriers method and their foam properties were measured by means of flowing method. The influences of dilational frequency and bulk concentration on surface dilational modulus and phase angle were expounded. The results show that the molecular interaction controls the nature of adsorption film during lower concentration range and the film behaves elastic in nature. During higher concentration range, the diffusion-exchange process controls the dynamic dilational properties and the surface film shows remarkable viscoelasticity. The foamability decreases with the alkyl chain increasing, while the trend of the foam stability is on the contrary.

Molecular Weight Dependence of Surface Dilatational Moduli of Poly(n-hexyl isocyanate) Films Spread at the Air−Water Interface

The surface dilatational modulus of the monolayer of a well-known semiflexible polymer, poly(n-hexyl isocyanate) (PHIC), at the air-water interface was measured as a function of the molecular weight of PHIC. In the dilute regime, the surface dilatational modulus of PHIC showed a linear response over a measured strain range of 5%-20%. The modulus could be well-separated into the elastic component E' and the viscous component E'', irrespective of molecular weight of PHIC. The E' and E'' components, respectively, are mainly attributed to static compression modulus caused by a change in the area covered by the PHIC monolayer and the semiflexibility of the PHIC chain. In the semidilute regime, the surface dilatational modulus of PHIC showed a nonlinear response to even 1% of strain due to the semiflexibility of the PHIC chain. Moreover, the surface dilatational modulus of PHIC depended on its molecular weight: at the smaller strain, the surface dilatational modulus of the high molecular weight PHIC was larger than that of the low molecular weight PHIC. The surface dilatational modulus of the high molecular weight PHIC increased with the surface concentration, whereas that of the low molecular weight PHIC remained constant. The molecular weight dependence in the semidilute regime is caused by the difference in chain entanglement between the high and low molecular weight PHICs at the air-water interface.

Comparison of the Surface Dilational Properties of Block Copolymers and Sodium Oleate Using the Oscillating Bubble Method

The surface dilational modulus of the star-like block copolymer AP432 and linear block copolymer L64 was studied using the oscillating bubble method at an accessible frequency range of 0.05 to 2 Hz, which was compared with anionic surfactant sodium oleate (C17H33COONa) and the influence of inorganic salt NaNO3 on the surface dilational modulus of AP432 and L64 was also studied. The results indicate that for AP432 and L64 the dilational modulus varies slightly, just decrease gradually with concentrations increasing and never reaches high values with the dilational frequency increasing, but the dilational modulus of C17H33COONa increases gradually with the increase of dilational frequency and pass through the maximum values with increasing concentration. The properties of AP432, L64, and C17H33COONa at the air/water interface were also observed by surface tension measurement. Based on these experimental results, a simple possible adsorption model of these molecules at air/water interface is proposed.

Hydroxypropylmethylcellulose at the oil–water interface. Part I. Bulk behaviour and dynamic adsorption as affected by pH

The bulk and oil–water (O/W) surface behaviour of four different commercial types of hydroxypropylmethylcelluloses (HPMCs) was studied at pH3 and 6. Dynamic light scattering (DLS) analysis of particle size distribution in HPMC water solutions indicated cluster formation at pH6 for most types. The dynamics of adsorption showed that the surface pressure ( π) values and the rate of adsorption/penetration were lower at pH3. The surface dilatational modulus of films at pH6 showed a continuous increase indicating a slow formation of a viscoelastic interfacial film. Nevertheless, at pH3 in most cases a viscoelastic film was not formed. The results obtained at the oil–water interface well correlated with DLS. Among all the HPMC studied, E5LV showed over all the best performance at this oil–water interface due to its low molecular weight and high decrease of interfacial tension. Its high hydrophobicity allows a strong tendency to interact at the interface to form elastic films.

Hydroxypropylmethylcellulose surface activity at equilibrium and adsorption dynamics at the air–water and oil–water interfaces

The surface behaviour of hydroxypropylmethylcelluloses (HPMC) was studied at the air–water (A/W) and oil–water (O/W) interfaces. At the A/W interface, the π-C isotherms were sigmoidal and presented inflexions as the HPMC bulk concentration increases, related with different structural patterns adopted by the biopolymer segments. The behaviour of these biopolymers at the O/W interface resulted different. No inflections in the adsorption isotherm were observed, denoting the absence of any change in the structure of the adsorbed monolayer. The order in the interfacial activity was different at the two interfaces. The dynamics of adsorption showed that the surface pressure (π) values and the rate of adsorption/penetration were lower at the O/W interface. Analogously to π values, the surface dilatational modulus was smallest at the O/W interface, however at long-term adsorption strong viscoelastic films are formed at the O/W interface.

Surface dilatational properties of whey protein and hydroxypropyl-methyl-cellulose mixed systems at the air–water interface

In this work we have studied the impact of the competitive adsorption of a whey protein concentrate (WPC) and three well characterized hydroxypropyl-methyl-cellulose (HPMCs), commercially known as E4 M, E50LV and F4 M, on the surface dilatational properties (surface dilatational modulus, E, surface dilatational elasticity, E d, and loss angle tangent, tan δ) of mixed films adsorbed at the air–water interface. The increase in E d values with adsorption time could be associated with biopolymer adsorption at the interface. The surface dilatational properties depend on the WPC and HPMC concentrations in the aqueous phase and on the WPC/HPMC ratio. Although the values of E d were mainly determined by HPMC at short adsorption times, for mixed systems with the lowest protein concentration (1 × 10 −4 wt%) the E d values were close to those of HPMCs, even at long term adsorption. The values of tan δ indicate the formation of adsorbed mixed films with high viscoelasticity, with a gel structure, which in turn should be attributed to the association of biopolymer molecules occurring at the interface. Only one biopolymer is the dominant one in the solid character of these mixed systems. HPMC at high concentrations slightly reduced the long-term solid character of the films confirming the existence of competition for the air–water interface as expected with two surface-active biopolymers with high molecular weight.

The effect of physiological conditions on the surface structure of proteins: Setting the scene for human digestion of emulsions

Understanding and manipulating the interfacial mechanisms that control human digestion of food emulsions is a crucial step towards improved control of dietary intake. This article reports initial studies on the effects of the physiological conditions within the stomach on the properties of the film formed by the milk protein (β-lactoglobulin) at the air-water interface. Atomic force microscopy (AFM), surface tension and surface rheology techniques were used to visualize and examine the effect of gastric conditions on the network structure. The effects of changes in temperature, pH and ionic strength on a preformed interfacial structure were characterized in order to simulate the actual digestion process. Changes in ionic strength had little effect on the surface properties. In isolation, acidification reduced both the dilatational and the surface shear modulus, mainly due to strong repulsive electrostatic interactions within the surface layer and raising the temperature to body temperature accelerated the rearrangements within the surface layer, resulting in a decrease of the dilatational response and an increase of surface pressure. Together pH and temperature display an unexpected synergism, independent of the ionic strength. Thus, exposure of a pre-formed interfacial β-lactoglobulin film to simulated gastric conditions reduced the surface dilatational modulus and surface shear moduli. This is attributed to a weakening of the surface network in which the surface rearrangements of the protein prior to exposure to gastric conditions might play a crucial role.

Surface Dilational Properties of <em>N</em>-Acyltaurate Amphiphiles with Aromatic Side Chains

The dynamic dilational viscoelasticity properties of N-2-(phenoxy)-tetradecanolytaurinate (12+B-T) and N-2-(4-ethylphenoxy)-tetradecanolytaurinate (12+2B-T) at the air/water interface were investigated by drop shape analysis and their foam properties were measured. The influence of time, dilational frequency, and bulk molar concentration on the surface dilational modulus and phase angle were investigated. A molecular interaction controls the nature of the film′s adsorption at the lower concentration range and the film is elastic in nature. At a higher concentration range, a diffusion exchange process controls the dynamic dilational properties and the surface film shows remarkable viscoelasticity. The addition of one ethyl group to the benzene ring enhances the molecular interaction which results in increases of the dilational modulus and foam stability.

Surfactant Mixtures for Control of Bubble Surface Mobility in Foam Studies

A new class of surfactant mixtures is described, which is particularly suitable for studies related to foam dynamics, such as studies of foam rheology, liquid drainage from foams and foam films, and bubble coarsening and rearrangement. These mixtures contain an anionic surfactant, a zwitterionic surfactant, and fatty acids (e.g., myristic or lauric) of low concentration. Solutions of these surfactant mixtures exhibit Newtonian behavior, and their viscosity could be varied by using glycerol. Most importantly, the dynamic surface properties of these solutions, such as their surface dilatational modulus, strongly depend on the presence and on the chain-length of fatty acid(s). Illustrative results are shown to demonstrate the dependence of solution properties on the composition of the surfactant mixture, and the resulting effects on foam rheological properties, foam film drainage, and bubble Ostwald ripening. The observed high surface modulus in the presence of fatty acids is explained with the formation of a surface condensed phase of fatty acid molecules in the surfactant adsorption layer.

The effect of pH on surface dilatational and shear properties of phospholipid monolayers

We have studied the surface dilatational (surface dilatational modulus, E, its elastic, E d, and viscous, E v, components, and loss angle tangent, tan θ) and surface shear (elastic, G′, and viscous components, η) characteristics of dipalmitoyl phosphatidylcholine (DPPC) and dioleoyl phosphatidylcholine (DOPC) monolayers, at 20 °C and at pH 5, 7, and 9. The stress response to compression–expansion sinusoidal deformation of the interface was measured in a modified Wilhelmy-type trough with two oscillating barriers and in a ring trough (with an isotropic area deformation), as a function of deformation amplitude and frequency. Measurements of surface shear rheology were carried out by using an automatic Du Noüy ring oscillatory apparatus. The monolayer structure and, especially, the conditions at which the monolayer collapses, determine the viscoelastic behaviour under dilatational and shear deformations of the monolayer and the linear response of the stress to area deformation. The non-linear viscoelastic behaviour of the interface has been associated with the phospholipid monolayer collapse. It was found that the dilatational modulus is not only determined by the interactions between spread molecules (which depend on the surface pressure or surface density), but that the structure of the spread monolayer also plays an important role. Surface shear rheology is more sensible to the effect of pH on surface rheological parameters of phospholipid monolayers than surface dilatational rheology. The surface shear characteristics for DPPC and DOPC monolayers were higher at pH 5, where attractive van der Waals interactions between phospholipid molecules take place, and were lower at pH 9, where electrostatic repulsions between phospholipid molecules weakened the monolayer structure. The surface dilatational and shear moduli were also higher for DPPC than for DOPC at every pH.